My rebbe in high school quoted his rebbe, Rav Aaron Soloveichik [sp?], who pointed out that Yom Haatzmaut does not conform with the normal way we make holidays. In all other holidays, we do not celebrate the war or the ability to fight, but we celebrate the next day when we could resume a Torah life in peace. Yom Haatzmaut was the changing of the flags, but not the end of the war. It just started a war with several Arab states. Nothing to celebrate.
You could also say that Yom Haatzmaut marks the end of the Shalosh Shivuot. If they accept that, then they are accepting that the Sh”Sh are/were Lemaaseh before Yom Haatzmaut.